When a compressed air installation which has previously been switched off and vented is switched on again, dangerous and damaging pressure surges can occur which lead to damage of cylinders arranged downstream. In order to prevent this, so-called starting valves for progressive build-up of pressure have been developed.
A starting valve of known construction is disclosed in EP-B-O 328 472 and comprises:
a housing with a first opening for coupling to a first component of the installation and a second opening for coupling to a second component of the installation, PA1 a channel connecting the two openings, PA1 a piston which is movable to and fro in the housing between a closed and an open position and has at least one first and one second pressure surface, wherein on the first pressure surface the pressure prevailing in the region of the first opening applies and on the second pressure surface the pressure prevailing in the region of the second opening applies, and wherein one of the two pressure surfaces has a direction of action which pushes the piston into the open position and the other pressure surface has a direction of action which pushes the piston into the closed position, PA1 means for maintaining a low flow of pressure medium--while the piston is in the closed position--from one opening to the other, PA1 and a spring element which biases the piston toward the closed position.
In this known starting valve the first pressure surface which urges the piston toward the closed position is in communication with the first opening at which the high pressure applies, so that the piston is initially brought into the closed position. By way of a bypass duct, which is disposed for example in the piston, the high pressure gradually passes to the other side and there acts in connection with the second pressure surface for the purpose of opening the piston. Since the second pressure surface is larger in terms of surface area than the first pressure surface, after a certain equalisation of pressure the closing force emanating from the first pressure surface and the spring element is overcome, so that the piston moves slowly into the open position.
Due to the pressure surfaces acting in opposite directions, this starting valve can be used for a wide range of pressures.
However, the principle of this known starting valve only allows one direction of installation in an existing pneumatic installation, since the high pressure may only apply at the opening which is in communication with the pressure surface which pushes the piston into the closed position. Therefore for the second installation variant a starting valve of quite different construction is necessary.